1. Technical Field
The present invention relates to a laminate adhesive and, more particularly, to a laminate adhesive that is useful for the manufacture of packaging materials for use in a variety of industrial fields, including packaging materials for food products, beverages, medicinal products and quasi-drags, and packaging materials for electronic components such as hard disks.
2. Description of the Prior Art
Composite films produced by laminating a plastic film, a metal foil such as an aluminum foil, a metallized film, and a silica evaporated film by the application of an adhesive have been widely used as packaging materials for use in a variety of industrial fields including packaging materials for food products, beverages, medicinal products and quasi-drags, and packaging materials for electronic components such as hard disks, up to the present date.
A two component ester urethane adhesive, using polyisocyanate in combination with polyester polyol, has been used most as the laminate adhesive for use in manufacturing these packaging materials, because of its excellent adhesion properties.
It is known, on the other hand, that polyester polyol produces by-products of a cyclic ester compound in the manufacture of it, and it has been hitherto believed to be hard to avoid the production of such by-products.
Industrially, when polyester polyol is produced in such a manner that polyester oligomer is synthesized by esterification reaction between polybasic acid and polyhydric alcohol or by ester exchange reaction between alkyl ester of polybasic acid and polyhydric alcohol, first, and then the polyester oligomer thus synthesized is subjected to condensation reaction, the number average molecular weight can be freely adjusted. However, in this technique, the ethylene glycol and propylene glycol that can be removed in reduced pressure must be used as the polyhydric alcohol. On the other hand, the polyester polyol used in the laminate adhesive uses an aromatic acid (usually a phthalic acid) as the polybasic acid, in order to provide the heat resistance to the laminate adhesive. However, the production of such a polyester polyol including the phthalic acid and ethylene glycol and/or propylene glycol inevitably produces the cyclic ester compound of the phthalic acid and the ethylene glycol and/or propylene glycol.
In recent years, there have advanced the studies on the substance that is eluted from packaging material into a content, which suggests the possibility that some low molecular weight compounds may be eluted from the adhesive into the content and the low molecular weight compounds thus eluted may attribute the cause, in part, to spoil the inherent properties of the content, such as the odor and taste of food products and beverages, or the inherent performances of the electronic components, though the cause and effect relationship has not yet been clarified.
It is the object of the present invention to provide a laminate adhesive that can suppress elution of a low molecular weight compounds of composite film into the content, so as to prevent the inherent properties or performances of the content from being spoiled by the low molecular weight compounds.